Explosive forming of metals employing a conical shock tube



W. S. FILLER July 20, 1965 EXPLOSIVE FORMING OF METALS EMPLOYING A CONICAL SHOCK TUBE Fil ed Dec- 27- 1960 INVENTOR. WILLIAM S. FILLER ado fink -piece only after the initial blast wave.

United States Patent 3,195,334 EXPLOSIVE FORMING 0F METALS EMPLOYING A CONICAL SHOCK TUBE William S. Filler, 302 Bradley Ave., Rockville, Md. Filed Dec. 27, 1960, Ser. No. 78,700

9 Claims. (Cl. 7256) The instant invention relates to metal working and pertains specifically to an apparatus and method for generating shock waves capable of effecting plastic deformation of metallic work pieces.

Shaping, cutting, bending, or otherwise working metals with the aid of explosives has been termed explosive metal forming" and is characterized by detonation of an explosive, customarily a high explosive, in close proximity to the metallic workpiece being treated. The resulting blast wave caused the desired deformation of the metal.

In;most high explosive metal forming operations an empirically determined quantity of high explosive is detonated at an empirically established distance from the workpiece in a gaseous or a liquid medium in some suitable container. In a significance which will be explained hereafter, the explosion is partly, often entirely, uncontrolled. Only a portion of the blast wave emitted from the detonation directly strikes the workpiece. Some portions of the blast wave are reflected from nearby surfaces such as the container walls, and strike the work- A very large portion of the blast wave propagates in direction away from the workpiece. When a liquid is the fluid propagating medium, a relatively long duration gas bubble pulse follows the brief blast Wave and provides additional pressure loads on the workpiece.

As a whole the complex series of direct pressure waves and reflected pressure waves are difiicult to control; their effects are diflicult to predict. As a result, numerous trial and error tests are required to establish proper quantitles of high explosives needed for a given job and the exact location to place the explosive relative to the workpiece. Thereafter, even modest changes in the dimensions or final shape of the workpiece require repetition of virtually the entire explosive test program to ensure satisfactory explosive metal forming. These and other drawbacks have slowed the development of explosive metal forming techniques, and have restricted their use to special instances where other methods failed completely or are so expensive as to be prohibitive. Nonetheless, explosive forming offers such considerable promise as to have been widly investigated and reported on in the literature, e.g., Special Report No. 474 which appeared in the June 15, 1959, issue of American Machinists, pages 127 through 138.

The instant invention is intended to provide both an apparatus and a method for generating and applying a controlled supersonic blast wave to a sharply restricted area in which is positioned a metallic workpiece backed up by the appropriately shaped die. Furthermore the positioning of the workpiece and die with respect to the explosive is achieved in a simple, precise and reproducible manner.

The practice of the instant invention may perhaps be best understood by considering what takes place in an unconfined air explosion effected in close proximity to the workpiece, and by comparison of the present invention therewith. When detonation occurs in air, the shock wave expands spherically outward from the detonation point. For metal forming, the skyward half, so to speak, of the expanding spherical shock wave is obviously a complete waste of energy. Even a large portion of the earthward half of the spherical wave is wasted. Indeed, only that portion of the spherically expanding blast wave contained within the spherical arc subtended by the workto compensate for the uncontrolled nature of most detonations by employing shaped explosives. Sheet explosives provide planular shock wave effects; linear elongated explosives (e.g. Primacord) provide cylindrical blast waves. Other special shapes also affect the explosive shock wave. At best, however, such efforts provide only partial control over the explosion.

The principal object of the instant invention is to provide both an improved technique and apparatus for gen erating a supersonic high explosive shock wave and for applying same in a controlled fashion to a workpiece in a concentrated and directed form. a

The basic phenomena underlying the instant invention are the recently discovered techniques for controlling the magnitude and direction of a blast wave. When an explosive charge is detonated at the apex of a cone the resulting shock wave expands within the confines of the cone in much the same manner as if the shock wave were the corresponding conical sector of the full spherical shock wave expanding from a larger uncontrolled explosion.

In brief, the blast from the explosive is confined to a small sector of a sphere by containing the explosive at the apex of a hollow metallic cone of suitable wall thickness and permitting the blast wave to expand only within the confines of its conical path. The impact strength of the blast wave on an object positioned in the cone may be many orders of magnitude greater than is available from the same quantity of explosive fired in an unconfined space. The amplifying effect of a cone of plane angle 0 with a quantity of explosive w fired at the apex, will result in a shock wave propagating down the cone with pressure and duration characteristics, essentially those of a weight of charge W defined by the expression where k is an elficiency factor varying from 0.3 to 0.6, depending on the cone design.

Fora more detailed description of the blast wave in a conical shock tube and the construction of such tubes, reference is hereby made to my copending application for Explosive Driven Conical Shock Tube, S. No. 78,794, filed December 27, 1960, and to the following published reports of the experimental work which relate to the instant invention and that of the above mentioned application: Measurements on the Blast Wave in a Conical Tube, volume 3, Number 3 (May-June 1960), The Physics of Fluids, pages 444-448; and to Design Characteristics of a Conical Shock Tube for the Simulation of Very Large Charge Blasts, NAVORD Report 6844, pub

lished October 1960 by the US. Naval Ordnance Laborathe workpiece. Furthermore, the shock wave will propagate through the surrounding fluid medium from the end of the cone with maximum amplitude along the direction of the axis of the cone. This will reduce the amount of explosive required for a given shock wave, provide greater flexibility in arrangements where explosive quan- 3 titles are small, as well as eliminate most spuriously. reflected shock waves. The quantitative and qualitative effects possible from practice of the instant invention can be illustrated by the exemplary instance of detonation in a 1 cone. Detonation of grams of high explosive charge creates a shock wave which strikes the area subtended by the cone terminus with virtually the force created by detonating 1 kilogram of the same high explosive at the same point, but in an uncontrolled man-.

ner.

For the practice of the instant invention, however, certain limiting features exist. First of all the detonation must be effected with what is known as a high explosive, largely because extremely high detonation wave propagation rates (e.g. 15,00020,000 ft. per second) are desired. Exemplary high explosives are pentolite (a 50/50 mixture of trinitrotoluene and penta-erythritol tetranitrate), dynamite, pressed trinitrotoluene, etc. Lower velocity explosives like gun cotton, gunpowder, explosive gas mixtures, etc., materials which can more accurately be termed propellants, are not contemplated except for the extreme and unusual special circumstances Where detonation of such propellant is so arranged that a high explosive shock wave results. Essentially the supersonic shock wave employed for the practice of the instant invention has a shock front pressure rise time of less than one microsecond, and its characteristics are unaffected by the quantity of explosive or mode of ignition.

Thus the practice of the instant invention is not comparable, for example, to what happens if a gun blast (or even a cannon blast) were directed at the workpiece. Compared to the practice of the instant invention the combustion wave of explodingpropellants is too low in intensity, or uncontrolled, or both, and may be subsonic and not a shock wave.

' characteristically, the practice of the instant invention requires an energy release region, i.e., a firing chamber, opening only into the shock tube. The blast wave generated in the firing chamber passes out through the generally tubular path provided by the shock tube.

Employment of a shock tube to so direct the supersonic blast wave has the advantage of making the wave controllable both in magnitude and character. Thus, for ex ample, the conicalshock tube is ordinarily preferred precisely because the supersonic blast or shock wave passing through so nearly duplicates the spherically expanding.

shock wave of an uncontrolled explosion. However, a cylindrical tube or even a converging cone can be employed instead of the expanding cone, and use ofsuch shapes is generally contemplated within the scope of the instant invention. Shock tubes with a geometry other than the expanding cone change the shape and duration of the supersonic shock wave through wall effects. As a result the shock waves received by the workpiece would be somewhat different from those of the presently recog-. nized uncontrolled explosion. Thus some control over the shock wave is obtainable through the geometry of the shock tube. other techniques of controlling the shock Wave willbe" For clarity, however, the discussiononwith'reference to the preferred conical shock tube, but

it should be understood that various control features and possible variations in utilization of the shock tube hereinafter given are as applicable to the cylindrical and the.

reverse cone shock tubes.

The principal control over the shock wave is, of course,

through predetermination of the energy input, namely the;

weight of the explosive charge. Shock tubes. may be constructed for explosive charges weighing less than one,

to or removing sections from an existing cone firing block configuration to obtain the desiredresult.

Another substantial control over the character of the shock wave is possible. through change in the medium inside the cone.- Air blasts have different characteristics than water blasts. Filling the cone with water causes the shock wave to exhibit all the characteristics of an underwater explosion, i.e., higher shock pressure, shorter amplitude than air explosions. In passing it should be noted how well adapted the instant invention is for underwater shots.- The presence of water or any other third medium or even a solid medium outside of the. cone fails to alter the amplification and the controlled characteristics of the blast wave. It is, however, necessary for underwater shots to use a wall thickness great enough to minimize radial elastic expansion of the cone wall and. prevent a resulting dissipation of the shock wave energy. Effectively a thicker walled cone must be employed for water shots than is necessary for shots with an air filled cone.

In any event, the actual construction of the cone can be varied for particular applications in predetermined fashion. Essentially, design and fabrication of the shock tube and firing chamber are well within the capability of workers in the weapons art. Thus the. thickness of the cone wall can be calculated on the basis of strength requirements needed to withstand the known peak pressures of the blast developed within the cone by the dynamite or other high explosive for'whatever amount used. Peak pressures of the shock wave in the cone may be determined through the combined use of standard shock pressure-distance data, the weight scaling laws for high we plosives, and the cone amplification equation mentioned earlier.

For the firing chamber, if desired, obsolete artillery pieces may be'converted to the. practice of the instant invention if more than several ounces of explosive are to be used. Actually the energy release or firingchamber receives the greatest blast shock. For repetitive use of the same firing block, certain design considerations have been found desirable.

While preferred firing block designs will vary depending on the specific application, cylindrical construction for the firing. chamber and use of a cylindrical explosive charge appear preferred.l Then significant plastic deformation of the firing block will not occur if a chamber diameter at least three times the explosive charge diam eter is employed; preferably, the diameter ratio should be about six to one in order to completely avoid plastic deformation. Also the diameter of the outside .wall of the firing block-should be 10 to 15 charge diameters to be adequate either for repeated firing or for a single shot ,where the charge is loaded directlyinto a close fitting hole and the block disposed of after each shot. For r e'peated firing block use, the explosive charge itself must, of course, be supported in spaced apart relation from the inside wall of the firing block with only a bare minimum of material between the explosive and the firing block wall. Thin Styrofoam and other resin foam rings have been found suitable for supporting and spacing purposes.

While the entrance to the cone constitutes the only real path for the blast Wave to exit from the firing chamher, there is otherwise no special geometric relationship between the exit of the firing chamber and the entrance to the shock tubes. Specifically, there is no needto make the firing chamber a geometric extension of-the' cone. The firing chamber may be cylindrical as preferred, or square, or any other suitable shape. If desired, if may even be larger than the apex end entrance of the cone. Regardless of theactual configuration of the explosive charge and the configuration of the firing chamber, the blast formsitself into the characteristic. expanding spherical shock wave by the time it is well'inside the shock tube. Once it is inside the shock tube and confined therein, the blast takes on the shock wave form dictated by the geometry of the shock tube, being, of course, a sector of a sphere for the conical shock tube.

Certain characteristics of a blast wave in the shock tube have no real parallel with the blast of an uncontrolled explosion. Thus it is Well known that the supersonic shock wave from an air explosion is largely reflected from the ground. Only a portion of the blast energy is actually transmitted to the surface impacted. However, when an equivalent size air detonation is effected at the apex of the conical shock tube, the reflected shock wave travels back up the cone to the apex. A second wave then propagates down the cone striking the workpiece to produce a second shock and a second reflection. This process will be repeated until the explosive energy has been completely dissipated. Increasing the cone length will increase the duration of the shock wave impact as well as the length of time between successive shock waves. In this fashion control can be exercised over the impact transmitted to the workpiece.

For further understanding of the instant invention reference is now made to the drawing which illustrates exemplary modes for the practice of the instant invention and wherein:

FIGURE 1 diagrammatically illustrates an apparatus for dishing an initially fiat workpiece;

FIGURE 2 diagrammatically illustrates an apparatus for applying an explosive blast wave to a cylindrical workpiece; and

FIGURE 3 diagrammatically illustrates how the apparatue of FIGURE 1 can be adapted to production line techniques.

As shown in FIGURE 1, the basic unit employed for the practice of the instant invention comprises a firing block and a shock tube illustrated by an expanding conical member 12, the two being joined by centering and clamping flange 14. While these two elements may be formed as an integral whole, they are illustrated as a pair of separate inter-fitting members. Suitable clamping devices may, of course, be employed to firmly but separately attach the cone to the firing block.

. A workpiece 16 is positioned at the terminus of cone 12, and directly behind the workpiece 16 is a suitable die 18. The space 20 between the workpiece 16 and die 18 may be evacuated to facilitate later confirming of the workpiece in the die. In the mode illustrated by the drawing, cone 12 rests directly upon the workpiece die assembly. The firing block 10 in turn rests directly upon cone 12.

FIGURE 1 further shows a high explosive charge 22. For small cone angles, e.g. 10, and a small workpiece 16, the explosive charge weight may be as little as that contained in a conventional blasting cap, on the order of a gram. Desirably, the explosive 22 is made up in cylindrical form so that it can be disposed centrally of cylindrical bore 24 in firing block 10, being spaced apart from the inside wall by spacer rings 26 (suitably Styrofoam). Electrical lead-s 28 are drawn through a narrow passageway in firing block 10 to provide for detonation of explosive member 22. At least a 3 to 1 diameter ratio of the firing block bore to the explosive is desirable.

In the operation of the mode of FIGURE 1 the firing block 10, the cone 12, the workpiece 16 and the die 18 may be assembled in air and fired as an air shot, so to speak, or preferably, submerged in water after assembly and fired as a water shot, care being taken during submergence to permit the cone to become substantially filled with water. If desired, the entire assembly may be put together underwater then fired as a water shot, or put together in air and the cone filled with water. In all cases the space 20 between die and work must not be filled.

Upon detonation of explosive 22 the blast wave initially expands in all directions as a supersonic shock wave, but because of the confinement and the control exerted by cone 12, the supersonic shock wave is concentrated to the conical sector of a sphere and passes down the length of shows the shock wave 30 at the instant of impact on workpiece 16. The shock wave strikes first at the center of workpiece l6 dishing out the center of workpiece 16 and then driving the marginal portions into the desired contact with the die member 18.

FIGURE 2 illustrates how the same cone and firing block can be applied to explosive metal forming of a cylindrical workpiece 40. In this instance the cone is positioned to discharge the shock wave into the central opening of workpiece 40. A suitable die member 42 is posi tioned around the workpiece, closing off the bottom and top of workpiece 40. Space 44 (preferably evacuated) initially exists between workpiece 40 and the die member 42.

While the direct impact of the entire shock wave (shown at 30) is not concentrated on the workpiece (as it is in the mode of FIGURE 1), the amplification factor still permits satisfactory attainment of the desired forming with relatively small quantities of explosive.

FIGURE 3 illustrates how the instant invention can be applied to production line techniques. A multiplicity of firing blocks 10 are mounted on the underside of an upper conveyor 50. Similarly a multiplicity of die members are mounted atop a lower conveyor 60, both conveyors traveling in the same direction, from left to right. In operation, the explosive 22 is placed in the individual firing block and the workpiece 16 is properly positioned on the die 18, at which time the space 20 between workpiece 16 and die member 18 is evacuated. Then the conveyors 50 and 60 bring together a firing block and the workpiece-die assembly to position A. A water filled cone 12 is disposed between firing block 10 and the workpiece-die assembly for detonation in position B. After the explosive has been detonated, cone 12 is removed at position C; then the workpiece 16 can be removed. There-' of the many basically different types of forming operations possible with this technique. For example, free forming might be accomplished by onlyclamping the metal to the cone directly. Also metal cutting, punching, cold welding, pressing powdered metal, and surface hardening, where these operations are performed through the application of forces generated by high explosives, can benefit from the use of this invention. It is therefore desired that, the present disclosure be considered in all respects illustrative rather than restrictive, within the scope of the appended claims.

What is claimed is:

1. A method for explosively forming metallic articles which comprises: positioning a metallic workpiece in juxtaposition with an appropriately shaped die and in position to be struck by a supersonic shock wave as hereafter described; detonating a high explosive charge in a firing chamber confined in all directions save at one exit area said firing chamber being several times the volume of said high explosive charge, generating thereby a supersonic shock wave; passing said shock wave through a generally tubular path commencing at the exit area of the I 3. The method of claim 1 .wherein said tubular-path is filled with liquid.

4. The method of claim 1 wherein the tubular path is filled with gas.

5. The method of claiml wherein the tubular path constitutes a conical structure flaring outwardly from the exit area of said firing chamber.

6. An apparatus for explosively forming metallic work-.

'pieces which comprises: a firing chamber adapted to contain a high explosive charge spaced apart from the chamber walls and having a shock tube joined at one end to said firing chamber; and .a die positioned at the other endof the shock tube, whereby a work piece placed between the'shock tube and the die will receive a directed.

means having secured thereto in spaced apart relation a plurality of firing chambers, each adapted to contain a high explosive charge spaced apart from the chamber walls; a second conveyor means having secured thereto in spaced apart relation a plurality of dies; a firing position wherein a die faces a firing chamber; and at least oneshock tube adapted to be .rernovably disposed between the paired firing chamber and dieat said firing position, whereby a workpiece placed between the shock tube and the the .will receive a directed high intensity supersonic shock wave from detonation of a high explosive charge in said firing chamber and whereby movement-of the two conveyor means allows workpieces to be formed sequentially on the various die members at said firing position.

References Cited by the Examiner UNITED STATES PATENTS 2,135,763 11/38 Nicholson.

2,923,204 2/60 Mohaup 89--1.02 2,935,038 5/60 Chatten 113-44 3,036,373 5/62 Drexelius 113-44 3,045,339 7/62 Callahan l13--421 OTHER REFERENCES Explosives Form Space Age Shapes, Steel, Aug. .25, 1958, pages 82-86..

Explosives, Materials in Design Engineering, vol. 49, No. 2, February 1959, pub., by Reinhold Pub. Corp, 430 Park Ave., New York, N.Y., pages 82-87.

CHARLES W..LANHAM, Primary Examiner.

NEDWIN BERGER, Examiner. 

1. A METHOD OF EXPLOSIVELY FORMING METALLIC ARTICLES WHICH COMPRISES; POSITIONING A METALLIC WORKPIECE IN JUXTAPOSITION WITH AN APPROPRIATELY SHAPED DIE AND IN POSITION TO BE STRUCK BY A SUPERSONIC SHOCK WAVE AS HEREAFTER DESCRIBED; DETONATING A HIGH EXPLOSIVE CHARGE IN A FIRING CHAMBER CONFINED IN ALL DIRECTIONS SAVE AT ONE EXIT AREA SAID FIRING CHAMBER BEING SEVERAL TIMES THE VOLUME OF SAID HIGH EXPLOSIVE CHARGE, GENERATING THEREBY A SUPERSONIC SHOCK WAVE; PASSING SAID SHOCK WAVE THROUGH A GENERALLY TUBULAR PATH COMMENCING AT THE EXIT AREA OF THE FIRING CHAMBER AND IMPACTING THE WORKPIECE WITH THE SHOCK WAVE AT THE TERMINUS END OF SAID TUBLUAR PATH. 